FRIDAY, March 23 (HealthDay News) -- People who live alone have a nearly 80 percent greater risk of depression than those who live with others, new research suggests.

Over the past three decades, the number of people living on their own in the United States has doubled, to one in three people.

The study included 3,500 working-age men and women in Finland who were followed for seven years. The researchers looked at the participants' antidepressant use; living arrangements; and psychosocial, sociodemographic and health risk factors, such as smoking, heavy drinking and lack of exercise.

In women, one-third of the increased risk of depression was linked to sociodemographic factors such as low income and lack of education, according to the study, which was published online March 23 in the journal BMC Public Health. The main risk factors for men included heavy drinking and a lack of support in the workplace or in their private lives.

"This kind of study usually underestimates risk because the people who are at the most risk tend to be the people who are least likely to complete the follow up," study author Laura Pulkki-Raback, of the Finnish Institute of Occupational Health in Helsinki, said in a journal news release. "We also were not able to judge how common untreated depression was."

More than half of the increased risk remains unexplained, the researchers noted. Possible factors may include feelings of alienation from society, lack of trust or difficulties caused by major life challenges.

Although the study found an association between living alone and depression, it did not prove a case-and-effect relationship.